Seismic surveys are performed from surface locations to obtain maps of the structure of subsurface formations. These surveys are in the form of maps (referred herein as seismographs") depicting cross-section of the earth below the surveyed region or area. Three dimensional ("3D") surveys have become common over the last decade and provide significantly better information of the subsurface formations compared to the previously available two-dimension ("2D") surveys. The 3D surveys have significantly reduced the number of dry wellbores. Still, since such seismic surveys are performed from the surface, they lose resolution due to the distance between the surface and the desired hydrocarbon-bearing formations, dips in and around the subsurface formations, bed boundary delineations, which is typically several thousand feet.
Surface seismic surveys utilize relatively low frequency acoustic signals to perform such surveys because such signals penetrate to greater depths. However, low frequency signals-provide lower resolution, which provides low resolution seismographs. High frequency signals provide relatively high resolution boundary delineations, but attenuate relatively quickly and are, thus, not used for performing seismic surveys from the surface.
Only rarely would an oil company drill a wellbore without first studying the seismographs for the area. The number of wellbores and the path of each wellbore is typically planned based on the seismographs of the area. Due to the relatively low resolution of such seismographs, wellbores are frequently not drilled along the most effective wellpaths. It is therefore desirable to obtain improved seismographs prior to drilling production wellbores. Additionally, more and more complex wellbores are now being drilled, the placement of which can be improved with high definition seismographs. Furthermore, relatively recently, it has been proposed to drill wellbores along contoured paths through and/or around subsurface formations to increase potential recovery or to improve production rates of hydrocarbons. In such cases, it is even more critical to have seismographs that relatively accurately depict the delineation of subsurface formations.
Conventionally, seismographs have been updated by (a) performing borehole imaging, which is typically conducted while drilling a wellbore and (b) by cross-well tomography, which is conducted while between a number of producing wells in a region. In the case of borehole imaging, a seismic source seismic source generates acoustic signals during drilling of the wellbore. A number of receivers placed on the surface receive acoustic reflections from subsurface formation boundaries, which signals are processed to obtain more accurate bed boundary information about the borehole. This technique helps improve the surface seismographs in piecemeal basis. Data from each such well being drilled is utilized to continually update the seismographs. However, such wellbores are neither planned nor optimally placed for the purpose of conducting subsurface seismic surveys. Their wellpaths and sizes are determined based upon potential recovery of hydrocarbons. In the case of crosswell tomography, acoustic signals are transmitted between various transmitters and receivers placed in producing wellbores. The effectiveness of such techniques are reduced if the wellbores are not optimally placed in the field. Such techniques would benefit from wellbores which are planned based on improved seismographs.
In the control of producing reservoirs, it would be useful to have information about the condition of the reservoir away from the borehole. Crosswell techniques are available to give this kind of information. In seismic tomography, a series of 3-D images of the reservoir is developed to give a 4-D model or the reservoir. Such data has usually been obtained using wireline methods in which seismic sensors are lowered into a borehole devoted solely for monitoring purposes. To use such data on a large scale would require a large number of wells devoted solely to monitoring purposes. Furthermore, seismic data acquired in different wireline runs commonly suffers from a data mismatch problem where, due to differences in the coupling of the sensors to the formation, data do not match.
The present invention addresses the above-noted problems and provides a method of conducting subsurface seismic surveys from one or more wellbores. These wellbores may be drilled for the purpose of conducting such surveys. Alternatively, permanently implanted sensors in a borehole that could even be a production well could be used to gather such data. The data from such subsurface surveys is utilized to improve the previously available seismographs. The improved seismographs are then utilized to plan the production wellbores. Borehole seismic imaging and cross-well tomography can be utilized to further improve the seismographs for reservoir management and control.